I’m not quite sure, but for one thing eggs are harder to get, and for another, this is my speculation, the game is coded in a way where pets are rarer than seeds, meaning you generally get less luck for the pet rather than the seed
I have okay luck on both things cuz i had gotten the griffin, spino and dragonfly! my luck with plants is 1 princess thorn 2 fossilights and 1 maple apple